Allowance for Loan Losses and Credit Quality Information - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013 SecurityLoan	Mar. 31, 2012	Dec. 31, 2012
Finance Receivable Transferred To Held For Sale [Line Items]
Percentage of core reserves factors in aggregate add up to	7.60%
Increase in calculated reserve percentage	2.00%
Interest income on impaired loans	$ 238,000	$ 93,000
Balance of troubled debt restructurings	22,800,000		22,000,000
TDRs in nonaccrual status	12,000,000		11,900,000
TDRs in accrual status	10,800,000		10,100,000
Troubled Debt Restructuring related reserves	1,900,000		936,000
Number of term loans modified in troubled debt restructurings	3
Increase in allowance for loan losses		38,000
Troubled debt restructurings charged off	$ 119,000	$ 795,000
Consumer Loan Commitments [Member]
Finance Receivable Transferred To Held For Sale [Line Items]
Term past due loans placed on nonaccrual status			90
Residential [Member]
Finance Receivable Transferred To Held For Sale [Line Items]
Number of TDRs that defaulted during period	0
Commercial Loan Commitments [Member]
Finance Receivable Transferred To Held For Sale [Line Items]
Number of term loans modified in troubled debt restructurings	1